A selective removal of a particular layer, such as for example a sacrificial layer, on a semiconductor substrate is a conventional measure in methods for manufacturing a semiconductor component, in particular in a method for manufacturing self-supporting structures. Thus, in the method for manufacturing a micromechanical sensor, a sacrificial layer is formed on the substrate, and on this layer another structured layer is deposited and structured. Subsequently, the sacrificial layer is selectively removed in order to expose the structures. In principle, the sacrificial layer can be removed using dry-chemical or wet-chemical methods.
German Patent Application No. DE 195 37 814 describes a sensor and a method for manufacturing a sensor in which on a silicon substrate a thermal oxide is first deposited, onto which a thin layer of highly doped polysilicon is then applied for use as a buried printed conductor. Another oxide layer is then deposited on the polysilicon layer, and for example a thick epipolysilicon layer is then deposited thereon. After this, the deposition and structuring of a surface aluminum metallization then takes place. Finally, the sensor structures that are to be exposed are etched free, preferably using a fluorine-based silicon deep etching method, described in German Patent No. DE 42 41 045. The exposure of the sensor element takes place using a sacrificial layer etching in which the oxide is typically removed under the sensor areas using media containing hydrofluoric acid, in a vapor etching method. A disadvantage of this underetching technique is that the oxide is removed not only under the sensor area that is to be exposed, but also above this area, and partially also under the polysilicon printed conductors, so that there is a danger of leakage paths and leakage currents. Protection of the oxide areas whose underetching is to be prevented, e.g., by protective lacquer, is possible only at a considerable expense, because vaporous hydrofluoric acid very quickly penetrates almost all practical polymer protective layers, and in addition can have a strongly corrosive effect.
The problems resulting from the use of vaporous hydrofluoric acid can be avoided by using a plasma etching method to carry out the underetching of the sensor elements. Thus, German Patent No. DE 44 20 962 describes a dry etching method in silicon is proposed in order to manufacture sensor structures in which, through a combination of anisotropic and isotropic plasma etching techniques, a subsequent wet-etching step or an etching in the vapor phase can be omitted. A separate sacrificial layer is not provided. All process steps can be carried out in a single plasma etching installation. For this purpose, first, again with the aid of the anisotropic deep etching method described in German Patent No. DE 42 41 045, the sensor structure is produced with vertical walls. Here, deposition steps in which a Teflon-type polymer is deposited on the side wall alternate with fluorine-based etching steps that are isotropic in themselves but are made locally anisotropic during the etching through the driving forward of the side wall polymer. Subsequently, a fluorine-based etching step is used to isotropically etch the silicon substrate until the silicon structure for the sensor element is completely exposed.
However, this method has two serious disadvantages. On the one hand, as a result of what is known as the “micro-loading effect,” narrow etched trenches are etched more slowly than are broad etched trenches, and this also holds for the speed of the subsequent lateral underetching; i.e., the underetching proceeds more slowly in narrow trenches than in broad trenches. On the other hand, the structures that are to be exposed are also attacked from their underside or floor. This has the result that structures that are surrounded by broad trenches have a smaller residual height than do structures that are surrounded by narrow trenches, which often results in non-reproducible and unsatisfactory mechanical properties of the manufactured sensor elements.
In order to enable a defined underetching, and thus to remove the disadvantages of the method described in German Patent No. DE 44 20 962, the method described in German Patent Application No. DE 198 47 455 A1 can be used. Here as well, a combination of anisotropic plasma etching for the structuring of the sensor elements and subsequent underetching thereof using isotropic gas phase etching or isotropic fluorine plasma etching makes it possible to do away entirely with hydrofluoric acid vapor. However, in this method an oxide layer that is only a few tens of nanometers thick is used as a separating layer directly above a polysilicon sacrificial layer in the area of the sensor structure that is to be produced. An epipolysilicon layer is deposited on the thin oxide layer and is structured using the anisotropic deep etching method described in German Patent No. DE 42 41 045. Here the thin oxide layer acts as a hard etch stop that is subsequently itself removed by a plasma etching using fluorine-rich etching gases such as CF4 or C2F6, under intense ion bombardment, i.e., using a high substrate bias voltage. Subsequently, the side walls of the sensor structures are coated with a Teflon-type polymer or an Si oxide thin layer as a protective layer, before the structured elements, made of epipolysilicon, are exposed by isotropic gas phase underetching, e.g., using ClF3, or also by an isotropic fluorine plasma underetching. Due to the high selectivity of this etching step of at least 200-300:1 relative to silicon oxide, the oxide layer on the underside of the structured elements, and possibly also on the side walls, prevents etching thereof on the rear side, or lateral etching attack.
However, the oxide on the rear side induces a pressure voltage and an upward curvature of the structure elements. From this there results a lasting influence of the oxide on the mechanical characteristics of the structure elements. In order to minimize this disturbing effect, the oxide layer should be as thin as possible, approximately 10 nm. In addition, practice has shown that the Teflon-type protective layer, or any type of protective layer, on the structure side walls is often not sufficient to prevent etching attack thereof during the isotropic gas phase or plasma etching step. Even an oxide layer in place of the Teflon passivation, applied in a cost-intensive deposition method, is not sufficient to ensure the necessary process reliability. After the exposure of the sensor structure, this oxide layer is then to be removed again at the structure side walls, for which reason the problematic effect of hydrofluoric acid or hydrofluoric acid vapor then finally applies.